Means for combining or merging two or more video signals into one video signal is known in the art. For instance, such technique is normally used in motion pictures or in television recordings for placing a foreground object or person in an environment that is different from the environment of the recording of the object or person.
Normally, such prior art means utilises a technique called colour-keying. In a simple form of colour-keying the foreground object or person is recorded, foreground recording, in front of a screen that is coloured by a single colour, usually blue or green. A second recording of the desired background is also made. Then, by checking the colour of the foreground recording, switching between two video signals representing the two recordings, respectively, may be performed based on the colour information of the foreground recording in order to combine the two recordings. When the colour of the screen in the foreground recording is detected the background video signal is switched into the video signal representing the combined video signal and when the colour of the screen is not detected in the foreground recording, the foreground video signal is switched into the video signal representing the combined video signals.
Colour-keying is a widely spread technique for combining two video signals and it may be used both on recorded and live video.
However, a colour-keying system requires that a specific colour in the foreground video signal is reserved as a replacement signal. Thus, this reserved colour may not be present in any part of the foreground that is not to be replaced by the background recording. This limits the number of allowed colours in the foreground recording and results in that a user has to make arrangements to prevent the reserved colour from turning up in the portions of the recording that should not be replaced.
In U.S. Pat. No. 5,566,251, describing a system for deriving a composite video image by merging video image data from a plurality of separate video signal sources, a technique different from colour-keying is used. In said system an operator designates an arbitrary target pattern, for example a logotype pattern on a billboard, in one video sequence. The system then monitors that video sequence continuously and in every video image of the video sequence that the logotype pattern is found the system replaces the logotype pattern with a video image data from another video signal source.
More specific, the system according to U.S. Pat. No. 5,566,251 searches for the logotype pattern and when found estimates the orientation, scale, perspective distortion etc. of the logotype pattern. These estimates are then applied to the video image data from the other video signal source in order to make the video image data of the other video signal source appear in the composite video image in a way corresponding to the appearance of the logotype pattern.
This system is very complex to implement and requires a lot of processing in order to detect the arbitrary logotype pattern and to estimate the orientation, scale, perspective distortion etc.